Recently, in various technical fields such as automobile, household products and electric appliance industries, demands for coated steel plates are increasing from the standpoint of productivity improvement and therefore, in the steel plate industry, energies are devoted to the development of plated steel plates which are excellent in both processability and corrosion resistance. Examples of such materials are aluminium containing hot dip galvanized sheet steel and the like. However, some of these plated steels cause troubles in connection with their chemical pretreatments for the purpose of improvements in scratch resistance, adhesion and corrosion resistance of the coating. For example, since a zinc phosphate film cannot effectively be formed on such plated steel because an amount of aluminium ions tend to accumulate in a treating bath and obstruct an effective film formation, a chromate coating is generally adopted as a preconstruction primer, but the scratch resistance of said primer can never reach the expected quality level.
On the other hand, in a chemical pretreatment for giving a phosphate coating, a chromate coating or the like, such miscellaneous expenses as equipment maintenance fee, treating bath maintenance fee, pollution control fee and the like, occupied in the total manufacturing cost are relatively high, and therefore, a further reduction in cost has been desired. Various coating compositions for precoat use have also been proposed for the improvements in processability and corrosion resistance of the coated steel plates. For example, the coating composition disclosed in Japanese Patent Publication No. 44569/77, comprises an epoxy base resin, an anticorrosive pigment and a lubricant and is claimed to be excellent in processability of the coating. However, as pointed out in the specification of said patent, for the obtainment of reliable scratch resistance, adhesion and corrosion resistance of the coating, it is essential to adopt a chemical treatment of the metal substrate. Thus, up to the present time, a primer composition for metallic material capable of resulting in a coating fully satisfying the latest quality requirements even when a chemical pretreatment is omitted, has never been provided.
An object of the present invention is, therefore, to provide a primer composition for metallic material capable of resulting in a coating with excellent scratch resistance, adhesion and corrosion resistance on a metallic material and especially galvanized iron, aluminium containing hot dipped galvanized iron, aluminium containing hot dipped galvanized sheet steel and the like.
An additional object of the invention is to provide a method for coating a metallic material with said primer composition and a top coat composition.
Further objects of the present invention are to provide a primer composition for metallic material which can evade the necessity of using a chemical pretreatment customarily adopted for the improvements in scratch resistance, adhesion and corrosion resistance of a coating and to provide a coating method using said primer composition.
Other objects of the invention shall be clear from the descriptions of the specification and claims.